/
do_either.erl
251 lines (204 loc) · 8.54 KB
/
do_either.erl
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%% @doc The Either Monad.
%%% @end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%_* Module declaration ======================================================
-module(do_either).
-behaviour(do_semigroup).
-behaviour(do_monoid).
-behaviour(do_foldable).
-behaviour(do_functor).
-behaviour(do_applicative).
-behaviour(do_traversable).
-behaviour(do_monad).
%%%_* Exports =================================================================
-define(API, [ % semigroup
append/2,
% monoid
mempty/0,
% foldable
foldr/3,
% functor
fmap/2,
% applicative
liftA2/2,
pure/1,
% traversable
sequence/1,
traverse/2,
% monad
bind/2,
do/2,
lift/1,
liftm/2,
then/2,
% either
either/3,
errors/1,
oks/1,
is_error/1,
is_ok/1,
from_error/2,
from_ok/2,
partition/1]).
-export(?API).
-ignore_xref(?API).
%%%_* Includes ================================================================
-include("do_internal.hrl").
-include("do_types.hrl").
-include("do.hrl").
%%%_* Code ====================================================================
%%%_* semigroup ---------------------------------------------------------------
-spec append(either(A, B), either(A, B)) -> either(A, B).
append({ok, B1}, {ok, B2}) -> {ok, do_semigroup:append(B1, B2)};
append({error, _}, {ok, B}) -> {ok, B};
append({ok, B}, {error, _}) -> {ok, B};
append({error, error}, {error, A}) -> {error, A};
append({error, A}, {error, _}) -> {error, A}.
%%%_* monoid ------------------------------------------------------------------
-spec mempty() -> either(atom(), _).
mempty() -> {error, error}.
%%%_* foldable ----------------------------------------------------------------
-spec foldr(fn(B, C, C), B, either(_, B)) -> C.
foldr(_, Acc, {error, _}) -> Acc;
foldr(F, Acc, {ok, A}) -> F(A, Acc).
%%%_* functor -----------------------------------------------------------------
-spec fmap(fn(B, C), Either :: either(A, B)) -> either(A, C).
fmap(F, {error, A}) when ?isF1(F) -> {error, A};
fmap(F, {ok, B}) when ?isF1(F) -> {ok, F(B)}.
%%%_* applicative -------------------------------------------------------------
-spec liftA2(Either :: either(A1, fn(B, C)), either(A2, B)) -> either(A1 | A2, C).
liftA2({ok, F}, Either) when ?isF1(F) -> fmap(F, Either);
liftA2({error, Reason}, _) -> {error, Reason}.
-spec pure(B) -> either(_, B).
pure(B) -> {ok, B}.
%%%_* traversable ------------------------------------------------------------
-spec sequence(either(A, applicative(B))) -> applicative(either(A, B)).
sequence({ok, B} = Either) -> do_traversable:sequence(Either, ?MODULE, ?Mod(B));
sequence({error, A}) -> {error, A}.
-spec traverse(fn(A, applicative(B)), either(A, B)) -> applicative(either(A, B)).
traverse(F, Either) when ?isF1(F) -> fmap(F, Either).
%%%_* monad -------------------------------------------------------------------
-spec bind(either(D, A), fn(A, either(B, C))) -> either(B | D, C).
bind(Either, F) when ?isF1(F) -> flat(fmap(F, Either)).
-spec do(either(A, B), [fn(B, either(C, D)) | fn(either(C, D))]) -> either(A | C, D).
do(Either, Fs) -> do_monad:do(Either, Fs).
-spec lift(fn(A, B)) -> fn(monad(A), monad(B)).
lift(F) -> do_monad:lift(F, ?MODULE).
-spec liftm(fun(), [either(_, B)]) -> either(_, B).
liftm(F, Eithers) -> do_monad:liftm(F, Eithers).
-spec then(either(A, _), fn(either(B, C))) -> either(A | B, C).
then(Either, F) -> do_monad:then(Either, F).
%%%_* either ------------------------------------------------------------------
-spec either(fn(A, C), fn(B, C), Either :: either(A, B)) -> C.
either(F1, F2, {error, A}) when ?isF1(F1), ?isF1(F2) -> F1(A);
either(F1, F2, {ok, B}) when ?isF1(F1), ?isF1(F2) -> F2(B).
-spec errors([either(A, _)]) -> [A].
errors(Eithers) when is_list(Eithers) ->
lists:filtermap(fun({error, A}) -> {true, A};
({ok, _B}) -> false end, Eithers).
-spec oks([either(_, B)]) -> [B].
oks(Eithers) when is_list(Eithers) ->
lists:filtermap(fun({ok, B}) -> {true, B};
({error, _A}) -> false end, Eithers).
-spec is_error(Either :: either(_, _)) -> boolean().
is_error({error, _}) -> true;
is_error({ok, _}) -> false.
-spec is_ok(either(_, _)) -> boolean().
is_ok(Either) -> not is_error(Either).
-spec from_error(A, Either :: either(A, _)) -> A.
from_error(_A, {error, A}) -> A;
from_error(A, {ok, _B}) -> A.
-spec from_ok(B, Either :: either(_, B)) -> B.
from_ok(_B, {ok, B}) -> B;
from_ok(B, {error, _A}) -> B.
-spec partition([either(A, B)]) -> {[A], [B]}.
partition(Eithers) when is_list(Eithers) ->
lists:foldr(fun({error, A}, {As, Bs}) -> {[A | As], Bs};
({ok, B}, {As, Bs}) -> {As, [B | Bs]} end,
{[], []}, Eithers).
%%%_* internal ----------------------------------------------------------------
flat({ok, {error, A}}) -> {error, A};
flat({error, A}) -> {error, A};
flat({ok, {ok, B}}) -> {ok, B}.
%%%_* Tests ===================================================================
-ifdef(TEST).
-include_lib("eunit/include/eunit.hrl").
append_test() ->
?equals(mempty(), append(mempty(), mempty())),
?equals({error, foo}, append(mempty(), {error, foo})),
?equals({error, foo}, append({error, foo}, mempty())),
?equals(pure(1), append(pure(1), mempty())),
?equals(pure([1, 2]), append(pure([1]), pure([2]))),
?equals(pure(1), append(mempty(), pure(1))).
foldr_test() ->
F = fun(A, B) -> A + B end,
?equals(2, foldr(F, 1, {ok, 1})),
?equals(1, foldr(F, 1, {error, 2})).
pure_test() ->
?assertEqual({ok, {ok, 3}}, pure({ok, 3})),
?assertEqual({ok, 3}, pure(3)).
lift_test() ->
F = fun(A) -> A + 1 end,
Lifted = lift(F),
?assertEqual({ok, 2}, Lifted({ok, 1})),
?assertEqual({error, rsn}, Lifted({error, rsn})).
liftA2_test() ->
F = fun(A) -> A + 1 end,
?equals({ok, 3}, liftA2({ok, F}, {ok, 2})),
?equals({error, rsn}, liftA2({ok, F}, {error, rsn})),
?equals({error, rsn}, liftA2({error, rsn}, {ok, 2})),
?equals({error, rsn}, liftA2({error, rsn}, {error, rsn})).
traverse_test() ->
F = fun(A) -> A + 1 end,
?equals({ok, 2}, traverse(F, {ok, 1})).
sequence_test() ->
?equals([{ok, 1}], sequence({ok, [1]})),
?equals({error, 1}, sequence({error, 1})).
liftm_test() ->
F = fun(A, B, C) -> A + B + C end,
?equals({ok, 4}, liftm(F, [{ok, 1}, {ok, 2}, {ok, 1}])),
?equals({error, rsn}, liftm(F, [{ok, 1}, {error, rsn}, {ok, 1}])),
?equals({error, rsn}, liftm(F, [{error, rsn}, {ok, 2}, {ok, 1}])),
?equals({error, rsn}, liftm(F, [{error, rsn}, {error, rsn}, {ok, 1}])).
bind_test() ->
FOk = fun(A) -> {ok, A + 1} end,
FError = fun(_) -> {error, reason} end,
?equals({ok, 3}, bind({ok, 2}, FOk)),
?equals({error, reason}, bind({error, reason}, FOk)),
?equals({error, reason}, bind({ok, 2}, FError)),
?equals({error, reason1}, bind({error, reason1}, FError)).
do_test() ->
Fun0 = fun() -> ?pure(3) end,
Fun = fun(A) -> ?pure(A + 1) end,
?equals({ok, 3}, do({ok, 3}, [])),
?equals({ok, 4}, do({ok, 3}, [Fun])),
?equals({ok, 4}, do({ok, 3}, [Fun0, Fun])),
?equals({error, reason}, do({error, reason}, [Fun])).
either_test() ->
F1 = fun(X) -> X end,
F2 = fun(_) -> 0 end,
?assertEqual(1, either(F2, F1, {ok, 1})),
?assertEqual(2, either(F1, F2, {error, 2})).
errors_test() ->
?assertEqual([], errors([])),
?assertEqual([1, 2], errors([{ok, 3}, {error, 1}, {error, 2}, {ok, 4}])).
oks_test() ->
?assertEqual([], oks([])),
?assertEqual([3, 4], oks([{ok, 3}, {error, 1}, {error, 2}, {ok, 4}])).
is_error_test() ->
?assertEqual(true, is_error({error, 1})),
?assertEqual(false, is_error({ok, 1})).
is_ok_test() ->
?assertEqual(false, is_ok({error, 1})),
?assertEqual(true, is_ok({ok, 1})).
from_error_test() ->
?assertEqual(1, from_error(2, {error, 1})),
?assertEqual(2, from_error(2, {ok, 1})).
from_ok_test() ->
?assertEqual(1, from_ok(2, {ok, 1})),
?assertEqual(2, from_ok(2, {error, 1})).
partition_test() ->
?assertEqual({[], []}, partition([])),
?assertEqual({[1, 2], [3, 4]}, partition([{ok, 3}, {error, 1}, {error, 2}, {ok, 4}])).
-endif.